1. Field of the Invention
The present invention relates to a semiconductor substrate for a production of semiconductor transistor devices or photoelectronic devices, and more particularly, to a semiconductor substrate comprising a single-crystalline semiconductor wafer substrate, a compound semiconductor epitaxial layer, and a strained layer superlattice (SLS) structure layer between the wafer layer and the epitaxial layer.
2. Description of the Related Art
An SLS structure layer is formed as a buffer layer between the semiconductor wafer substrate, such as a GaAs wafer and Si wafer, and a compound semiconductor layer, such as an InGaP layer and GaAs layer, for the following reasons:
(1) The SLS buffer layer reduces dislocations in the grown compound semiconductor layer by preventing an extension of threading dislocations from the substrate into the compound semiconductor layer, due to the strain field in the SLS; and
(2) The SLS buffer layer allows an epitaxial growth of the compound semiconductor layer having a different lattice constant from that of the substrate by absorbing a lattice mismatch due to an alternate expansion and contraction of strained thin layers of the SLS structure layer.
For example, a GaAs grown on a Si (GaAs/Si) substrate instead of a single-crystalline GaAs wafer substrate is produced by using the SLS structure layers to enlarge the wafer size, increase the mechanical strength and thermal conductivity, and reduce costs, compared with the GaAs wafer substrate, a lattice constant of GaAs being larger than that of Si by about 4% (e.g., cf. N.El-Masry et al, "Defect Reduction in GaAs Epilayers on Si Substrate Using Strained Layer Superlattices", Mat. Res. Soc. Sympo. Porc. Vol. 91, 1987, pp. 99-103). Furthermore, it is often necessary to form (grow) a compound semiconductor layer having a different or a same lattice constant from or as that of the wafer substrate for light emitting devices or lasers having a desired emission wavelength, while reducing the number of crystal defects such as dislocations.
Currently, there is a tendency to overestimate the SLS buffer. Namely, in practice, although an SLS buffer uses a pair of compound semiconductor layers having a large lattice mismatch therebetween, a compound semiconductor epitaxial layer growth on the SLS buffer has a high density of dislocations larger than that of a wafer substrate, by five or six fold, and when a graded layer is additionally grown between the wafer substrate and the SLS buffer, the compound semiconductor epitaxial layer also has a high density of dislocations. Therefore, the effect of reducing the number of dislocations in the SLS buffer is very low.
To obtain a required dislocation reducing effect, preferably the compound semiconductor substrate including the SLS buffer is heat-treated at a high temperature (i.e., an annealing out of excessive dislocations).
A conventional SLS buffer, however, is not proof against a heat-treatment (annealing) at a high temperature, ever for a short time, and a decay of the SLS structure by an interdiffusion of the alternating thin layers thereof occurs. Therefore, although the number of dislocations is reduced, the decay of the SLS structure causes variations in the characteristics of the devices, for example, an increase of a full-width at a half maximum (FWHM) of an LED, and prevents further annealing or thermal cycling improvement.